Railway signal system



L. ESPENSCHIED RAILWAY SIGNAL SYSTEM 21, 1919 4 Sheets-Sheet 1 FiledNov.

INVENTOR L. ESPENSCHIED RAILWAY SIGNAL SYSTEM Filed Nov. 21. 1919 '4Sheets-Sheet INVENTOR L. ESPENSCHIED RAILWAY; SIGNAL SYSTEM Filed Nov.21, 1919 4 Shets-Sheet s INVE NTOR Dec. 1924. 1,517,549

L. ESPENSCHIED RAILWAY SIGNAL SYSTEM Filed ubv.'21, 1919 4 Shets-Shet 4JNVENTQR sure following.

Patented Dec. 2, 1924;

LLOYD ESPENSCHIED, OF'QUEENS VILLAGE, NEW YORK.

RAILWAY SIGNAL SYSTEM.

Application filed November 21, 1919. Serial No. 339,658.

To all whom it may concern:

Be it known that I, LLoYn EsrnNsoHrnD, a citizen of the United States,residing at Queens Village, in the county of Queens and State of NewYork, have invented certain Improvements in Railway Signal Systems, ofwhich the following is a specification.

This invention relates to railway signaling systems. More particularlyit provides means for indicating, as in an engine cab, the condition ofthe track ahead. I

The invention is directed at promoting safety in railroad operation. Oneobject is to provide a signaling system which does not requiresectlonalizing the track and the provision of signaling mechanisms alongthe right of way as does the block type of signal system, but whichenables the equipment to be entirely localized and placed aboard alocomotive. A further object is to increase the certainty on the part ofthe engineer of correctly seeing the signal by having the same set up inthe engine cab instead of along the right of way. .A still furtherfeature of the present system is that it is not only capable ofindicating the condition of the track ahead, but also of controllingautomatically in accordance therewith the headway of the train and ofbringing it to a stop under conditions of danger. Further and moredetailed objects of the invention will beevident from the disclo- Tofacilitate a comprehensive understanding of the invention there is givenfirst an outline of the broader principles of the method and meansinvolved.

In accordance with the present invention the railway signal iscontrolled by sending out wave energy from the signal location throughtransmission circuits in the right of way and causing this outgoingenergy to effect, in accordance with traflic conditions, the return ornon-return to the signal of energy for .controlling the signal.According to the invention therefore there are two separate and distinctenergy transmissions-an outgoing wave and'a returning wave, the latterbeing dependent on the former and the control of the signalbeing'efi'ected thereby.

In the preferable forms of embodiment, of the invention,'the track railsare utilized as the conductors for both the outgoing and i the returningenergy waves, suitable appatical or acoustic properties.

ratus being employed to discriminate between the waves at the signallocation. If

the signal is to be given in the engineers cab, the wave energy isimpressed on the track preferably by a source of energy located in thelocomotive so that expensive wayside equipment is minimized.

The returning energy wave may be set up in any suitable manner. Themethod particularly described herein utilizes as a fundamental principlethe reflection of electric energy Waves, but it is to be expresslyunderstood thatthe invention is not limited thereto. This method has theimportant advantage, however, of requiring no intrumen talities in thetrackway or on the train for originating the returning energy wave, aswill be described more in detail hereinafter.

When electric wave energy is impressed on an infinitely longtransmission circuit or line whose electrical constants; inductance,resistance and capacity per unit of length of the line, aresubstantially the same or uniform throughout, the waves of energyimpressed thereon flow uniformly along the line, generally becomingsmaller and smaller in amplitude because of the attenuat-mg effect ofthe circuit. If the length of the line is finite, however, the wavesupon reaching the distant terminal of the line will encounter anirregularity (unless apparatus having the identical electrical constantsis associated therewith), which irregularity has the effect of causingpart of the current wave to be reflected .to the sending end of theline. This phenomenon is similar to that of the reflection of light andsound waves when such waves traveling in one medium, encounter anothermedium of differentop- An echo for example, is produced when the soundwaves traveling in one medium, such as air, encounter another medium ofdifferent acoustic properties, such as a wall. Part of the sound wavesenter the wall and are absorbed thereby and the remainder is reflectedinto the air, the ratio between the absorbed and reflected portionsdepending upon the difference in the sound transmitting pro erties ofthe two media, or in other. words, t e degree of the irregularityencountered by the waves. So it is with waves in an electrictransmission circuit or line. At the end Of the line there is areflection of the current waves, unless the apparatus connected theretohas the same impedance properties, and the amount of reflection dependsupon the degree of irregularity eucountered by the waves. The reflectedwaves return to the sending end of the circuit where they may bedetected by suitable apparatus. If the circuit is very long,electrically speaking, the reflected waves, however, are so dampened outowing to the attenuation of the line, that they become practicallynegligible, just as an echo cannot be heard if the reflecting wall istoo far away. If the circuit is made shorter and shorter the reflectedwave grows stronger and stronger in intensity as it returns to thesending end, similarly as an echo grows louder as the reflecting wall isapproached.

In one form of embodiment of the invention the track rails properconstitute the conductors of the transmission circuit or line and asuitable source of current is provided to send out electric waves overthe said circuit. When the waves encounter an irregularity in the railcircuit, as for example a break in the circuit, as caused by a brokenrail, or a short circuit, such as is caused by the wheels and axles of atrain, they are reflected toward the transmitting end, where suitableapparatus is provided for their detection. If the irregularity is farahead.

' the waves are very weak, indicating that it is safe to proceed overthe track at maximum speed; if nearer, the reflected waves are strongerindicating that caution-should be exercised, and if close by, the saidwaves are of such intensity as to indicate that it is unsafe to proceed.J It will thus be seen that the present invention is characterized bythe feature that there are two distinct flows of energy,

namely an outward flow which exists irr\- spective of the trafficconditions ahead of the train and a return flow which is set up onlyunder certain traffic conditions. as for example, the condition of ashort circuit by a train in advance or a broken rail. this return flowof energy being segregated by suitable apparatus from the outward flowand being utilized to control the indications of the signal.

The invention thus diflers broadly from the prior systems in whichelectrical energy generated on the train, whether alternating or direct,is passed through an ordinary circuit extending along the right of way,which circuit is controlled by opening and closing it, as by a switch,or by the presence of a second train on the track. Even if inconsidering such a system the artificial viewpoint is adopted, that theexistence or nonexistence of the current (according for instance, to theclosed or open position of aswitch) is caused by reflection at theswitch, then it must be recognized that the reflection reflected wavesas wellas a break in the rails). Consequently, with reflection occurringfor both the open and closed positions of the switch there is nocondition in the circuits of the previously proposed systems whichcorresponds to the clear track condition of the present invention, noprovision being made for preventing the return flow of energy to thesignal for one condition of signaling, as for example, the clear trackcondition. As a matter of fact the prior systems do not even recognizethe existence of a reflected wave and consequently make no provision forsegregating the reflected and outgoing energy waves, much less use thereturning wave, as distinguished from the outgoing wave, for controllingthe signal.

Another manner in which the present form of embodiment of the inventionmay be explained consists in considering the entrant impedance of thetransmission circuit of the line, this impedance being the ratio of theelectro-motive force impressed upon, to the current entering, thetransmission circuit. ,Where the circuit is electrically long i. e.,many wave lengths long, and of corres ondingly high attenuation theentrant impedance for analternating current is substantially independentof the condition at the other end of the circuit, as to whether openedor closed, and is determined solely by the linear constants of thetransmission line. For such an electrically long circuit the entrantimpedance is called the characteristic impedance and is defined by (Ta cwhere R is the series resistance er unit length (say per thousand feet),gr is the shunt conductance per unit length; L the inductance and C thecapacity per unit length;

and j is the vector notation of complex algebra to indicate a phasedifference of 90;

and w indicates 21: times the frequency.

smaller value and will have a reactance component.

Where the circuit 'm not long electrically,

puted from the formula the effect at the sending end of the distant"termination will change the value of the the ratio of =Z. The normalimpedciably affect the current at the originating end. If we take anelectrically long circuit and greatly reduce itslength, there comes apoint at which the reflected wave is returned to the sending end insufiicient amplitude to appreciably affect the resultant sending currentand to thereby alter the entrant iInpedance. dif We continue to shortenthe length of the circuit the reflected wave becomes an increasinglygreater-factor until at relatively short lengths the entrant impedanceis determined largely by the'reflection eifect and in turn by theterminal impedance of the circuit.

The manner in which the characteristic entrant impedance of a trackcircuit varies with the length thereof is illustrated in Figure 3. Thecurves show the effect of moving a short circuit, such as that caused bya train on a track, from a distance out in'toward the point ofobservation.- The impedance values plotted are of the resistance R andthe reactance X(Z=R+jX) for 50,000 cycles impressed metallically uponthe track circuit. Starting at say eight miles out and decreasing thedistance, the imped ance expressed in terms of resistance and reactance,will be noted to gradually deviate in an oscillatory manner from thenormal clear track value until, when the short circuit comes withina-distance of about one mile, both the resistance and reactancehave beengreatly altered. The values of the impedance curve of Figure 3 have been'com- P /(R -l 'iu L) (GT-?EOE where Z is the resultant entrantimpedance, Z is the 'infinite line or in" this case clear trackimpedance 1, the lengthof the circuit and P is the propagation constant.

C: The term tanh PLrepresents the effect of the refiectedwave at thesending end.

The following values per thousand feet have been assumed, for purposesof illustration, in the computations for the curve of i Fig. 3. R:0.02ohms; G=0.00l ohms; L=0.00045 hen; (3:2.3X10' fds.

It is the impedance characteristic of a" transmission circuit asoutlined above which is utilized in the present invention for indicatingthe proximity of two railway trains, one to the other. The sect on oftrack between the two trains is treated as an electric transmissioncircuit, and one train, for instance. the oncoming train, is used asthe? transmitting endof the circuit, and the other train, the itrainahead, is used, by virtue of its effect in short circuiting the rails ofthe track, as the distant termination of the circuit. The strength andphase relation of the wave reflected from the train ahead back to theoncoming train is used as an indication of the d'stance apart of the twotrains.

Thus so long as the separation is so greai that the reflected wave,received back at the oncoming train, is of inappreciable amplitude, asignal on said train is set indicating a clear track. 7 When a lesserseparation exists, giving an appreciable reflected wave at the oncomingtrain, and requiring cautious proceeding this condition may be made toindicate caution and, if desired, to slow down the oncoming train. Whena still lesser distance and one involving danger of collis on, separatest "e two trains, this condition is made to ind cate danger and to effectautomatically the stoppage of the oncoming train.

One of the features of the present invention is the use of relativelyhigh frequencies for limiting by virtue of the high attenuation therange or distance ahead to which the signaling system is responsive. Bythe use of a high frequency the signaling mechanism is renderedunresponsiveto .and the'engineer is not bothered by trains so far aheadas to not constitute a danger. The exact frequency employed isdetermined by the constants of-the circuit employed and by the distancewithinwhich it is'desired to operate a signal.

The frequency to be employed for any given condition of signaling isdetermined by computing the reflection effect by formulae as indicated'above, or by measure coordinating such results with the characteristicsof-the terminal apparatus. It should be understood that while theinvention in its' preferred form employs frequencies in the radiofrequency range, it is notlimited in respect to frequency.

Having explained the general principles involved in the invention and.having indicated the method of applying them in a railway signalingsystem, there are hereinafter described several specific embodiments ofthe invention.

Figure 1 illustrates diagrammatically one form of the entire signalingsystem.

FigureZindicates the manner of impressing the alternating E. M. F. uponthe track circuit and has reference to the entrant impedancecharacteristic of the track circuit which as indicated in Figure 3 isthe basis of operation of the invention.

Figures 4, 5 and 5 show details of means for making connection betweenthe track circuit and the local apparatus.

Figure 6 illustrates an alternative form of the local circuitarrangements.

Figure 7 illustrates the use of an alarm and control circuit.

Figure 8 shows a localcircuit arrangement adapted to a furtherdevelopment of the general scheme.

Figure 9 illustrates means for indicating the condition of the trackimpedance by a method diiferent than that employed in the precedingfigures.

Figure 10 discloses the manner in which the signal transmission circuitmay be superimposed upon the power supply circuit of an electric railwaysystem.

Referring now to Figure 1, A representsthe signaling station at what hasbeen referredto as the sending end of the track circuit. The apparatusof this station may be located on the locomotive of a train as indicated in the figure. T represents the two rails of the track formingthe two conductors of the electric circuit. A represents the Ftrainahead. At station A alternating current of high frequency is impressedthrough coil C upon the circuit represented by the rails of the track asbridged by the wheels and axle am, of the locomotive. 'B represents aWheatstone bridge circuit for effecting a balance between the receiverR, and the generator G, and comprises the ratio arms a and b. the arm a:corresponding to thebranch of unknown impedence and the arm 3corresponding to the rheostat arm of the ordinary Wheatstone bridge. Thegenerator G may be any of the well known sources of high frequencyalternating current such as the alternator, the Poulsen are or thevacuum tube oscillator. The receiver R may be anytype of instrumentoperative directly or indirectly from alternating current such as a hotwire or dynamometer type A. C. instrument or a D. C. instrument operatedfrom a thermocouple or a rectifier. The track circuit is included, bymeans of coil C, in the an arm of the bridge circuit. The network N ofarm y is adjusted to be equal in impedance at the frequency of thegenerator to the coil ing distance.

and 0 which tune these arms to the frequency employed, i. e., counteractthe positive reactance, and thereby increase the sensitivity of thebridge circuit. The a: and 3 arms are thereby made of equal impedancesand balance the bridge circuit B.

The operation ofthe system is illustrated in Figure 1 is as follows:Alternating currents generated at G pass equally through the ratio armsras and by of the bridge. cause no difierence of potential across andtherefore do not aflfect the receiver R. A part ofthe current in coil Cis induced into the rail circuit T. Assume that the bridge circuit isbalanced for a clear condition of the track circuit, in which case thereceiver R is not operated by generator G and indicates a clearcondition of the track ahead. The current induced in the track circuitpropagates to A the train ahead, and is reflected back to train A by theshort circuiting effect of one or more of the axles of train A Thedesign of the system is made such that when the separation of the trainsis so great as to constitute a clear track condition, then theattenuation of the track circuit is so great that the ,wave reflectedback is too weak to aflect the' receiving apparatus at station A When,however, the separation between the trains is so short as to bedangerous the reflected wave is returnedto station A, with amplitudesuflicient to operate the receiver R. The course of this reflectedcurrent is through coil C and arm w to the bridge circuit B and thenceto the receiver R. The balanced bridge circuit serves thus todiscriminate between the outgoing and the reflected current; it preventsthe former from having any effect on the receiver but it permits thelatter to enter the receiver and cause it to respond. It will beunderstood that other suitable discriminatory devices may be employed ifdesired.

Another and more direct way of expressing this operation is to say thatthe bridge circuit B becomes unbalanced and causes the generator G tooperate the receiver R, when the train ahead is closer than a certainlimit- As the separation between trains is still further reduced, theunbalance of the bridge circuit becomes greater and the deflection ofthe indicating instrument correspondingly larger. It will be noted thatas the unbalance becomes greater the transmission efiiciency between thegenerator and receiver increases, this efiiciency being nil when thebalance is perfect, as will be readily understood. As illustrated, theindicator is marked in three steps, one indicating clear,

. another danger, and a third stop. It will be understood that anymethod of indication and any type of signaling device may be used.

The manner in which current is induced from coil G into the trackcircuit is explained more fully in reference to Figure 2. Thecombination of coil C and the. three-quarter loop formed by the rails,and the short circuiting axle'aan' form a transformer, the primary ofwhich is coil C and the secondary of which is the said three-quartertrack and axle circuit. The E. M. F., E, is impressed upon the track asa transmission circuit, through this track-coil transformer. Consideringas terminals the points on the track at which the M. F.,. E, isimpressed, then the characteristic impedance Z, looking along the trackis defined by the ratio %a where I, is the alternating current enteringthe track circuit at the terminal points. v 7 1 This impedance Z isdetermined solely by the linear constants of the circuit aspreviand isthe 'impedance'which obtains at the sending end of the track circuitwhen there is no train ahead or when the train is so far ahead that thereflected wave is damped out. This impedance will be appreciated to be avector quantity having both magnitude and phase angle and resolvabletherefore into a' resistance and a reactance component Z :R |-jX Thisresistancecomponent of course 1s not the same as the ordinary ohmicresistance of the circuit. In the case'illustrated, where the frequencyis high, its magnitude is determined more largely by the 'ratio of theinductance to the capacity of the circuit than by the'ohmic'resist'ance.

This-characteristic impedance Z represents, then, the ratio of voltage'to current intensity obtaining in the energy wave which is launchedintothe circuit and is determined solely by the linear constants of thecircuit'and not by the termination, whether open or short circuited.When there is a train present on the track causing a short' circuit,or'when the track circuit is open as at a broken .rail, then a reflectedwave is set up at that'point and this reflected waveupon return to thesending end is superimposed upon the, then' incident wave and changesthe entrant impedance of the track circuit from that ofthe-characteristic impedance to a resultantdmpedance. If the point ofreflection is so far ahead as not to constitute danger, then thereflected wave is so highly attenuated as not to appreciably afl'ect thesending end impedance and the 'from the curves of the figure.

point of reflection, such as a as the distance out to the point ofreflection is changed, is shown in graphical form in Fig. 3. This figurecontains two curves, one the resistance component and the other thereactance component of the resultant ,entrantimpedance (looking into thetrack circuit from the sending end). These two components combinedrepresent the entrant impedance, which is a vector quantity, themagnitude and phase of which progressively change as. the distance ofthe point of reflection changes, because of the progressive change inmagnitude and phase of the returned wave. In other words, this impedancerepresents the ratio of a resultant electromotive .force to a resultantcurrent (in the track circuit and not in the receiver 'circuit itself).a

The resistance component R of the entrant impedance (Z=R+jX) representsthe ratio of those components of the resultantvoltage and current waveswhich are in phase.v The reactance component X represents the ratio ofthose components of the voltage and current waves which are out ofphase. j denotes this 90 phase relation in accordance with theconvention of complex algebra. Inasmuch as each of these is the resolvedcomponent of factors which are themselves the resultant of the outgoingand returned wave action, each may be expected to change in magnitude asthe distance to the point of reflection is changed. This fact isapparent, When the short circuit is as far out as 10 miles, for

example, the 'efl'ect of the reflected wave is suppressed and theimpedance is merely the characteristic impedance of about 400 ohms,without any reactance (for the case illustrated) as'shown by the curveR. As the distance is decreased the reflected wave, as

the strength with which it is received increases, gradually modifies theimpedance.

Both the res1stance and the reactance curves are caused to deviate fromtrue characteristic impedance, more and more asthe distance isdecreased. It will be noted that when the resistance. component-is amaxi mum or a minimum, the reactance component is zero and that when'the reactance is a maximum or minimum, the resistance component is thatof the characteristic resistance (about 400 ohms); This alternation of'the two components is caused by the progressive change in the phasebetweenthe outiao anced circuits;

to effect the signalling on the locomotive behind. This is accomplishedby providing a .balanced circuit on the locomotive which balances thereceiver against thetransmitter for the condition of characteristicimpedance, so that when this condition obtains, as it does when thetracks are clear for a considerable distance ahead, the circuit isbalanced. Then for the condition where the track immediately ahead isoccupied, the entrant impedance in the track circuit is materiallyaltered and the circuit is unbalanced, thus giving the control signal.

The above explains the operation of the invention in terms of impedanceand bal- While this is a convenient method of dealing with the subject,it should be remembered that impedances are simply numerics representingthe ratio of voltage to current waves and that the actual entitiesinvolved in the operation of the system are the wave transmissionsthemselves. An energy wave stream is continuously propagated out alongthe track of the locomotive and, when the track ahead is clear, thisenergy transmission passes out in effect never to return. When, however,the track ahead is occupied or a rail is broken, then the energy streamis reflected and partially returned to the sending end and enters thereceiving device independently of the outgoing stream upon which it issuperimposed in the track circuit, and effects the control of thelocomotive. It will thus be appreciated that the essential positive factof the invention re sides in the establishment of the transmission ofenergy out and back, and that the impedance diagram in Fig. 3 is simplya convenient way of showing the result of this out and back transmissionin such terms as to link up with the wave propagation viewpoint thebalanced circuit method of treatment. c In accordance. with one methodof operatmg this signal system, the bridge circuit of Figure 1 isbalanced for the normal clear track impedance condition. As theimpedance deviates from the normal with decreased train separation asillustrated in Figure 3, the unbalance between the source Gand thereceiver R of Figure 1 increases until the receiver is actuated andeifects the desired signal. The exact point on the impedance curve ofFigure 3, corresponding to a certain separation, at which the signaloperates is determined by the ratio of the receiver' sensitivity to thegenerator power and may be adjusted for any desired value. Theindicating instrument may be calibrated in terms of clear for normalimpedance caution for a deflection of the receiver corresponding to theimpedance change caused by the separation being reduced down to say 2 or3 miles, and danger corresponding to a larger deflection caused by thelarger impedance variation incident to the separation being reduced tosay one mile. This ability to thus definitely relate the indicationin'the locomotive cab with the distance ahead of another train is anportant part of the present invention.

One of the problems involved in the successful operation of this type ofsignal'system is that of making suitable connection between the trackand the local circuit on the locomotive. The means of Figures 1 and 2for so doing operate on the principle of electromagnetic induction. TheE. M. F., is induced in part directly in the rails and in part in theforemost axle of the locomotive. Other embodiments of electromagneticinductors are shown in Figures i and 5.

In Figure 4 the inducing coil C is so designed and disposed in relationto the track circuit as to impress the E. M. F. directly upon the railsalone. Coil C is divided into two sub-coils, 1 and 2, for the two railsrespectively. These coils are wound about a- U-shaped core, which maybe'air or otherwise suitably laminated iron, and the cores are sodisposed as to have a maximum in ducing eflect upon the trail in a wellknown manner as indicated. The inducing coil may be related solely withthe axle as indicated in Figure 5. This is a less desirable disposition,however, because of the shunting effect of the trucks behind. It will beunderstood that any such track connecting means as described above maybe located to the rear of one or more axles of the locomotivepr train,in which case it is necessary to insulate the preceding axles andthereby prevent them from short circuiting the track and shielding theterminal circuit from changes in the condition of the track impedance.The connection with the track may be made through a pair of plates, oneplaced close to each rail and forming a condenser therewith, the othersurface of the condenser being the rail itself, and the dielectric ot'the condenser being the air space between the plate and the rail.The'vehicle circuit is thus associated with the track circuit byelectrostatic rather than by electromagnetic induction. The trackconnection may also be made conductively by brushes bearing upon therails, wheels or axles or by contact through the journals as illustrateddiagrammatically in Figure 5". The difli; cu ties involved in makingconnection with the track inductively, through either mutual inductanceor capacity, .is in securable changes in the track impedance to bedetectedin the local circuit of the locomotive. The use of relativelyhigh frequencies is a feature of the invention in this respect inthat-it facilitates the transfer of energy across what is a transformerof relatively large magnetic leakaga'.

. Figure 6 illustrates a terminal circuit arrangement broadly similar tothat of Fig-- ure 1, but differing in respect to the bridge circuit.design, the source of the high frequency alternating current, and thereceiving mechanism. The bridge circuit B of Figure 1 is replaced by oneof a different form in- 3b cuit, the period of oscillation of which isdetermined by .the inductance 12 and capacity and which can. be changedby varying condenser 14, for instance. ;The high frequency alternatingcurrent so generated is taken off through a secondary ooil 113which,together with the other two. coils 12 and 13, forms atransformer11. The detecting branch of the bridge circuit is related with thebridge through the third Winding of the transformer 5. This circuit istuned by inductance 6 and capacity 7 to the frequency of the localsource. The sensitivity of the entire circuit to variations in impedancethrough the coil C is increased by means of an amplifier 8 of the wellknown vacuum tube form. The indicating instrument 9 -may be of thedynamometer type, similar to a wattmeter. It may be operated in eitherof .two wayseither't he two coils are connected in series, as in commonpractice, and excited from the output of amplifier 8, or one coil isexcited from the oscillator, as shown, through circuit 10. In thislatter case, the indication of the instrument becomes a function of'thephase displacement of the return transmission. Selec tivity is alsoimparted by the tuning of the detector branch referred to above, and bythe tuning of the as and yarms of the bridge circuit proper. I

In the embodiments described above "the signal is given merely as adeflection of an pedance unbalances and to actuate signal alarm orcontrol mechanism.

Figure 7 illustrates the use of relays actuated by a detecting devicefor operating alarm and control circuits. The left hand portion of thecircuit, coil 5 in particular,

' is intended to fit with the detector branch of the bridge circuit ofFigure. 6. The .received currents are tuned by inductance 6 andcondenser 7 are amplified by the vacuum tube amplifier 8, are rectifiedby the vacuum tube detector 17, in the output circuit of which are therelays 18, 19 and 20. Relay 1.8 is the most sensitive and is adjusted tooperate on arelatively small current corresponding to the moderateimpedance irregularity caused by a train ahead coming within the cautionzone. Its operation closes the circuit 21 and rings the alarm bell 22,or performs some other signaling opera- 'tion. Relay 19 is iven amarginal adjustment whereby it will respond to a current of some greatervalue corresponding to a nearer approach of the train ahead into thedanger zone for instance, and closes the circuit 23 of anelectro-magnetic device 24. The armature 25 is drawn in part way andeffects a reduction in the speed of the train, as by partial operationof a steam valve and associated, air brake valve 26. Upon the furtherapproach of the train to the danger point, the detector current is.

further increased, relay 20, set to operate on this larger current, isactuated, circuit 27 is closed and the armature 25 is drawn in furtherand the steam and air valves are completely operated thus bringing thetrain to a stop.

Referring back to the impedance curves of Figure 3, it will be seen thatwithin the distance at which the impedance irregularity becomesapparent, the resistance and re' actance curves oscillate, back andforth across the impedance value corresponding to a clear track. Thesecurves oscillate with an approximately 90 phase displacement however, sothat at no point. within the range in which they oscillate,i'. e.,within say 6 miles, does their combined value equal I the normal entrantimpedance of the transmission. circuit (the track). To further insureagainst the possibility of a false safesignal being given within thedanger zone two currents of different frequencies may,

be employed simultaneously, and the giving of a safe signal madedependent upon their being affected similarly. Means for doing this areillustrated in Figure 8, which is generally similar to Figure 6. Gr andG are the two sources of high frequency,

alternating current, which sources are tuned indicating instrument. Suchsignal-indicat-i by inductance-capacity combinations 30 and ing meansmay be supplemented or replaced by relay devices set to'operate atdefinite iming a transfer of energy suflicient tov en- 31 respectively.The ac and y arms are tuned for two different frequencies by theaddition of tuning circuits 33 and 34: re-

1 and either of these currents will operate the indicating circuit suchas that of Figure 4. For all distances within the zone of operation ofthe signaling system the terminal circuit will be unbalanced to both ofthe two frequencies thus insuring operation.

The circuits described above are of a balanced type and operate byvirtue of the effect upon the condition of balance of changes in thetrack impedance. The operation is described as one wherein the circuitis balanced for a clear track and unbalanced for an obstructed track.This ad justment may be reversed, i. e., the circuit may be normallyunbalanced and the responding instruments normally actuated for theclear track condition, the balance being improved as the separationbetween trainsis decreased, the improvement becoming such when theseparation has reached a certain minimum as to release, say the alarmrelay, and sound an alarm, and the balance being further improved with afurther reduction in separation, resulting in the release of controlrelays and finally in the stoppage of the engine in the some generalmanner as described above. This method of operation corresponds to theclosed circuit method of control employed in signaling circuitsgenerally, while the reverse method first described above corresponds tothe open circuit arrangement of ordinary signaling circuits. This closedclrcuit operation is illustrated as applied to another embodiment of'the invention following.

Figure 9 illustrates a circuit arrangement operating on a principledifferent from that of the balanced types of circuits described above.The method of operation is that of causing the changes in the trackimpedance to efi'ect changes in the period of oscillation of anoscillating circuit and to cause in turn the resulting changes in thefrequency to control the receiving or indicating devices. Theoscillating circuit .is of a type well known in radio engineering. Thevacuum tube'amplifier 15. is connected'on its input side across one-halfof a coil C and on its output side with the second half of said coil asillustrated. A condenser 14 is. bridged across the circuit and functionsto determine in combination with the inductance of coil C, the period ofoscillation.

The coil of the oscillator is related to the track circuit inductivelyand in this case 'is the coupling coil itself. Current generated by theoscillator is transmitted through transformer 40 across anti-resonantcircuit 4142 to the amplifier 8 and thence to the detecting andindicating apparatus which may be generally similar to that of Figure 7.

I The operation of the circuit arrangement is as follows: For theimpedance value corresponding to clear track conditions, the oscillatorgenerates a current of definite frequency. This current is passedthrough transformer 40 and amplifier 8 and normally holds in operationfor instance, the

relay responding devices. The circuit bridged across the input of theamplifier 8' effects a marked reduction in the current supplied to thedetector 17 and associated responding devices 18 and 19'. These devicesare so adjusted as to release at certain predetermined reductions in thecurrent, so that-they are actuated in accordance with the change infrequency suffered by the oscillator. The release of relay 19, forinstance, releases relay 44 which trips oif an indicating or controlelement 4:5. The method of operation here described is that of theclosed circuit type. The open circuit type of operation is of courseequally applicable.-

Figure 10 illustrates the manner in which an alternating current railwaysignaling circuit may be superimposed upon the power supply circuit foran electric railway. While this arrangement is applicable to railwaysignaling systems generally it is especially useful in the reflectedwave type of system because the power supply circuit is a more eflicientand constant signaling transmission circuit than is the the trackcircuit, described above in relation to Figure 2. Referring to Figure10, 51 and 52 represent the two rails of the track which are, of course,well grounded to form the groundreturn of the power supply circuit. Thethird rail, trolley or other power supply conductor is indicated as 53while 54 is the shoe, pantagraph or other sliding contact member. Thepower receiving circuit is through the ath from shoe 54, 55 throu h aninserted lter 56, the controller 57, t 0 motor ormotors M and back toground through point 58 and the wheels and track. This power circuit maybe D. C. orlow fre quency A. C. The filter 56 is provided for excludingthe higher frequency signaling currents from the power receivingterminals 55, 57, M, 58. It consists of a plurality of sections ofseries inductance and shunt capacity and where the signaling and powerfrequencies are greatly different may be reduced to a simple inductancecoil.

The signaling terminal circuit connecting with no corresponding with thecircuit w of the previous figures, is bridged across the terminals and58 of the power receiving circuit through a filter 59. This filterfunctions to exclude thelow frequency or D. C. power currents whilepassing the higher frequency signaling currents. It may take the formillustrated of a filter which transmits freely frequencies above anassigned limit, such as 100 cycles per second and which substantiallyexcludes frequencies below this limit. In one well-known form thisfilter consists of a plurality of sections of series condensers andshunt coils. When the power and signaling frequencies are sufficientlydifferent it may be simplified to a mere single series-connectedcondenser,

In thus applying the signaling circuit to the power supply system, thefilter 56 or its equivalent may be provided in all shoeconnectingcircuits of a train for the purpose of improving the signalingtransmission efficiency of the power circuit. This provision in itselfwould tend to prevent the power terminal on the train ahead, forinstance,

from short circuiting or otherwise affecting the impedanceof the thirdrail circuit sufficiently for signaling by the reflected wave principle.It is therefore desirable to either omit the filter 56 from one or moreof the shoe-circuits not employed in the signalin circuit, or to add thefilter 59 of the signa branch circuit and to short-circuit the signalterminal circuit 02 thereby to short circuit the third rail at thesignaling frequency and at a point considerably behind the signalinggenerator and receiver. An-' other way of insuring that a train willmaterially affect the third rail circuit impedance is to purposely notfit together the impedances of the terminal and of the trans missionthird rail circuit. Still another way of so doing is to operatesuccessive trains on different signaling frequencies and to provide ananti-resonant circuit, 60, across the signaling terminal circuit. Thisis tuned to the freshort circuit the transmission circuit with respectto all other trains while selectively opening this short for itself thusenabling it to get in on the circuit without material sacrifice intransmission eificiency.

It will be understood that the invention permits of many variations andpermutations in the manner of practicing it Without departing from thescope and spirit thereof as defined in the following claims. V

What I claim is: a

1. The method of railway signaling which consists in alteringtheimpedance of a transmission circuit comprising both track rails inaccordance with the length of track between a train and a source ofdanger producing corresponding changes in the balance of a circuit andoperating thereby a signal device.

2. The method of railway signaling by the reflected wave principle whichconsists in impressing upon the transmission circuit a frequencysufficiently high as to attenuate and substantially suppress the effectof the reflected wave for distances greater than #4 that to which it isdesired that the receiver be responsive.

3. In a railway signaling system the combination of a source ofalternating current, a receivertherefor and means whereby the effect ofsaid source upon said receiver is determined by the entrant impedancecondition of the track.

4:. In a railway signaling system, the combination of a source ofalternating current, a vacuum tube type of receiver, balanceddiscriminating means included between said source and said receiver andmeans whereby the effect of said source upon said receiver is determinedin accordance with traffic. conditions.

5., In a railway signaling system, the combination of a source ofalternating current, a vacuum tube type of receiver, high frequencydiscriminating means included between said source and said receiver andmeans for controlling the efiect of said source upon said receiver inaccordance with the position of a vehicle.

6. In a railway signal or control system, a source of alternatingcurrent, a receiver therefor comprising an amplifier, a vacuum tubedetector and a relay, said source and relay being at the same locationand means continuously responsive'to trailic conditions tinuouslycontrolled by said source of alternating current in accordance with saidconditions.

7. In a railway slgnal or control system,

a source of alternating current, a vacuum tube detector thereforadjacent said source,

a plurality of relays, said source and relays being at the samelocatlon; and means continuously responsive to traffic condltions forcontinuously governing the operation of said relays by said source inaccordance with said conditions. I

8. In a railway signaling system, a transmission circuit extending alonthe right of way, said circuit being contro led by traffic conditions soas to change its impedance proressively in accordance with the distanceroin a source of danger, a source of high frequency alternating currentfor said circuit, a signal device, said device comprising a vacuum tube,means for so associating said circuit, source and device, that saiddevice is non-responsive to said source, and means for rendering saiddevice responsive to said source under certain predetermined impedanceconditions of said circuit.

9. In a railway signaling system, a circuit responsive to trafiic, asource of current therefor, a device for balancing said circuit, saiddevice having an impedance equal to that of the said circuit for certaintrafiic conditions, so that the balance between the said circuit anddevice is upset for either an open or a short-circuit condition of saidcircuit, a

signal translating device and means responsive to an unbalance betweensaid circuit and balancing device to cause an operation of said signaldevice.

10. A railway signaling system comprising a circuit responsive totraffic conditions, the impedance of said circuit being subject tochange progressively in accordance with the distance from a source ofdan er, a source of high frequency current there or, a device responsiveto direct current, and means comprising a vacuum tuberectifier wherebysaid device is so associated with said circuit as to be responsive tothe high frequency currenttherein.

11. A railway signaling system comprising a circuit responsive totrailic conditions,

the impedance of'said circuit being subject to change progressively inaccordance with the distance from a source of dan er, a vacuum tubeoscillator associated witi said circuit for im ressing thereon currentof predetermined requency, a device responsive to direct current andmeans for associating said devlce with sand circuit and rendering thesame selectively responsive to current from said oscillator, saidmeanscomprising a freuency-selective device, and means for rectiying saidcurrent.

12. A railway signalling system compris mg a signal circuit including asignaling device, a traflic controlled means, a source of alternatingcurrent ener com rising means whereby energy is sent rom t e location ofthe signal into said traflic controlled means, said traflic controlledmeans having the property of sending electric energy back to said signallocation under certain traflic conditions so-asto enter said signalcircuit and effect an indication of the signal, and means for preventingthe outgoin energy from passing direct y from the sai source lnto thesignal circuit without passing through the traflic controlled means.

13. The system as in claim 12, in which the said signal circuit and thesaid source are located ona railway vehicle, and in which inductivemeans are provided for eifecting' the transfer of energy to and from thetrafiic controlled means.

14. The combination claimed in claim 12, in which said trafliccontrolled means comprises a circuit extendin way forcarrying both theoutgoing and returning energy transmissions.

15. The system as described in claim 12, in combination with meanswhereby the return wave is rendered ineflective to control the signalwhen the distance between the signal location and the point at which thereturn wave is originated exceeds a minimum desired for safety.

16. The system as described in claim 12, in which said alternatingcurrent circuitin the right of way comprises the rails 'of the track.

17. The system as described in claim12, in which said signal device is,located on the train and in which the said alternating current circuitextends ahead of the train.

18. A railwa trafiic control system com- 'prlsing fa signa devicelocated on a railway vehicle, an alternatingmurrent transmission path,said path comprising the rails of the track ahea of the vehicle, meanswhereby electric ener is sent but from the vehicle throu h saitransmission path, and means where y certain trafiic conditions cause anelectric irregularity in the. said path, said'irregularity causing thereflection of energy along said path toward said signal to effect acertain indication thereof. y 19. The system as described in claim 18,in combination with means whereby the irregularity is introduced intothe track rail transmission path by the short circuiting of the saidpath as by the wheels and axles of a preceding car or train, or by theopening of the said path, as b a broken rail.

combination with a balanced circuit arrangement for associating thesignal with the said alternating current circuit, whereby the effect onthe signal of the outgoing wave .is neutralized but not the effectthereon of the returning wave.

21. A system for controlling railway traffic, com risingl a si a1device, a transmission pat in t e rig t of way, a circuit arrangement,comprising a circuit for balancing the said path, and for associatingthe si al device with the'said path, and means w ereby electric ener issent from the location of the signalt rough said path, said balancingcircuit -h avmg an impedance along the right of.

20. The system 'as escribed in claim 12, in

x it

I said path, so that either an opening or a short-circuiting of saidpath will bring about an unbalance and a control of the signal.

22. The system as described in claim 12, in which the electric waveenergy impressed on the said circuit in the right of way comprises aplurality of waves differing in frequency.

23. In a railway signalling system, a source of energy," a receiving.device,.means for associating said source and device, said meansincluding a balanced circuit arrangement for rendering said receivingdevice under normal traffic conditions non-responsive to said source,said balanced circuit arrangement including a wayside circuit whereby achange in the impedance of said circuit will upset the balance of saidcircuit arrangement and cause an actuation of the receiving device.

24. i In a railway signalling system, a

source of energy, a signalling device, a wayside circuit responsivetotraflic conditions, and means for operatively associating said source,circuit, and signalling device, said means comprising an impedancearrangement for balancing the impedance of the wayside circuit undernormal traific conditions.

25. In a railway signalling system, a source of energy, a signallingdevice, a wayside circuit responsive to traflic conditions,

and means for operatively associating said source, means comprising an.impedance device having an impedance equal to the characteristicimpedance of said wayside circuit, whereby either an opening or aelosing of said circuitwill cause an unbalance of said circuitarrangement so as to bring about an actuation of said signal device.

26. In a railway signalling, system, a source of high frequencyalternating current, a. signal device, said device comprising avacuuin't'ube, said source and device being located on a railwayvehicle, and trafli'c controlled ineans for governing the response ofsaid device to said source.

' 27 In -'a railway signalling system, a

' source of high frequency. alternatin current, said source comprisingan e ect'ron circuit, and signalling device, saidtube oscillator, asignal device, said device comprising an electron tube detector, saidsource and device being located on a railway vehicle, and trafliccontrolled means for governing the response of said signal device tosaid source. I

-' 28. In a railway signalling system, a source of current and .asignal, said source and signal beingllocated on a railway ve hicle, saidsource comprising a generator of high frequency alternating current, anelectron tube interposed between said source and said signal, andtrafiic controlled means for governing the response of said signal tocurrent from said source.

29. In a railway signalling system, a source of current and a signal,said source and signal being located on a railway vehicle, said sourcecomprising a generator of high frequencyaltern'ating current, anelectron tube interposed between said source and said signal, andtrafiic controlled means for governing the response of said signal toouring a wayside circuit.

30. In a railway signalling system, a source of current and a signal,said source and signal being-located on a railway vehicle, said sourcecomprising avacuum tube oscillator for generating high frequencyalternating current, an electron tube interposed between said source andsaid signal, and trafiic controlled means for governing 'the response ofsaid signal to current from said source, said means comprisinga" waysidecircuit.

: 31. In a railway signalling system, a,

source of current and a signal, said source and signal being located ona railway vehicle, said source comprising a vacuum'tube oscillator forgenerating high frequency alternating current, a detector and a vacuumtube amplifier, and traflic controlled means for governing the responseof said signal to current from said source, said means comprising awayside circuit.

-In testimony whereof, I have signed my name tO iZlllS s e'cificationthis 19th day of November, 191 .1 LLOYD ESPENSCHIED. f

